Bajada del Diablo, Patagonia, Argentina: The impact of a split comet?

ACEVEDO, R.D.; ORGEIRA, MARÍA JULIA; PREZZI, CLAUDIA; RABASSA, J.; CORBELLA, H.; PONCE, J.F.; MARTÍNEZ, O.; VÁSQUEZ, CARLOS; SUBÍAS, I.; GONZÁLEZ, MAURICIO; ROCCA, M.

Londres

Congreso; 74rd Annual Meteoritical Society Meeting (2011); 2011

Meteoritical Society

An impact origin has been proposed for the almost 200 circular structures found in Bajada del Diablo (42° 46´ to 43° S and 67° 24´ to 45? W), Patagonia, Argentina, some of them partially obliterated by erosion or sediment accumulation, which range in diameter from 100 meters up to 400 meters. The circular structures have been identified on two rock types: the Quiñelaf eruptive complex (Miocene basalts) and Pampa Sastre Formation (Pliocene basaltic conglomerate). Geological and geomorphological strong evidences of involvement on both geologic entities more over the Pleistocene pediment gravels and sands has been interpreted as the cause of the impact but lack either pieces of the impacting body like meteorites or definitive signs of shock metamorphism. Only samples of breccias randomly spread were collected. In neither case, however, was detected there any evidence of coesite. Most important diagnostic elements collected so far are tiny Fe-Ni lawrencite-bearing microspheres of 300 microns in average diameter, presumably originated by the impact, buried under post-impact sedimentary coverage. To strengthen the hypothesis of impact through the geophysical survey magnetometric record of magnetic anomalies were performed. The magnetic anomalies show a circular pattern with magnetic lows in the circular structure?s floors. Furthermore in the circular structure?s rims, high-amplitude, conspicuous and localized (short wavelength) anomalies are observed. Such large amplitude and short wavelength anomalies are not detected out of the circular structures. For all used frequencies, the electromagnetic profiles show lower apparent electrical conductivities in the circular structure?s floor, while the rims present notably higher values. Curvature attributes, analytic signal, horizontal gradient and Euler solutions were calculated for the magnetic data. 2.5 D magnetic models were developed across each one of the studied circular structures. Our results suggest that in the circular structure?s floors up to 12 m of Pampa Sastre conglomerate would have been removed. On the contrary, the circular structure?s rims exhibit high-amplitude, localized magnetic anomalies and higher apparent electrical conductivities, which would be related to the anomalous accumulation of basalt boulders and blocks remanently magnetized. The fact that such high-amplitude anomalies are not present out of the surveyed circular structures supports this hypothesis. The geomorphological, geological and geophysical features of the studied circular sructures can only be explained by means of an extra-terrestial projectile impact. Even Fe-Ni microspheres with lawrencite and breccias have been found inside craters, nevertheless, the absence of direct evidences of a disintegrated asteroid opens the possibility to consider the event of a split comet with an ice nucleous hitting the patagonian surface in Pleistocene times.